This research proposal aims at elucidating the potential tumor-suppressor function of the transcription factor interferon regulatory factor 4 (IRF4) in B-cell malignancy. The development of B cells is tightly controlled by the coordinated expression of master transcriptional regulators. Aberrant expression of these regulators due to genetic alterations can disrupt ordered B-cell development and contribute to oncogenic transformation. Our long-term goal is to identify the roles of IRF4 in B-cell development, since deregulation of the biological programs controlled by IRF4 has been linked to the pathogenesis of several types of B-cell tumors. IRF4 can act as an oncogene in tumors of late B-cell stages, including multiple myeloma, whereas it has been identified as a potential tumor suppressor in chronic lymphocytic leukemia (CLL), a malignancy of quiescent mature B cells. Such opposing pathogenic roles of a single transcription factor imply the existence of fundamentally different biological programs controlled by IRF4 in different B-cell subsets. Thus, there is a need to elucidate the stage-specific function of IRF4 in order to understand its diverse pathogenic roles. While there is accumulating information on the function of IRF4 in the late stages of B-cell development, the role of IRF4 in quiescent mature B cells remains largely unexplored. Our goal here is to identify the role of IRF4 in mature B cells in order to understand the potential tumor-suppressor function of IRF4 in the pathogenesis of CLL. We have obtained preliminary evidence in a mouse model suggesting that IRF4-deficiency causes aberrant migration and homing of mature B cells, possibly due to hyperactivation of NOTCH2 signaling. The objective of the proposed work is to determine the molecular mechanisms by which IRF4 controls trafficking of mature B cells through regulation of NOTCH2 activation in order to understand the biological consequences of IRF4 deficiency in B-cell malignancy. Our central hypothesis is that alterations in the balance of the transcriptional network established by IRF4 and NOTCH2 may disrupt the normal migration and homing properties of B cells and thereby contribute to lymphomagenesis by aberrantly positioning transformed B cells in a lymphoid microenvironment that supports survival. To accomplish the objective of this application, we will elucidate the molecular mechanism by which IRF4 regulates NOTCH2 activation. In addition, we will determine the biological program controlled by NOTCH2 in IRF4-deficient B cells by performing a genome-wide identification of transcriptional targets. We will also identify the role of the IRF4-NOTCH2 interplay in cell migration in vivo by exploring the consequences of NOTCH2-inhibition in IRF4-deficient B cells, using a therapeutically active NOTCH2-inhibitory antibody. The rationale for the proposed research is that elucidating the functional interplay between IRF4 and NOTCH2 in normal B-cell physiology and its disruption in malignancy has the potential to provide the basis for developing innovative therapeutic strategies targeted at CLL and other B-cell tumors where NOTCH2 hyperactivation has been associated with tumor-promoting roles.